Journal of the American ChemicalSociety
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substituents, the alkynyl groups are held adjacent.' The analogy with bis(phenylethyny1)benzeneI is given further credence by the conversion of 2 ( M = Pt; R = R' = Ph; X = CI) in moist, acidic chlorobenzene into the unsymmetrical diphosphine complex 5 in good yield, presumably via initial electrophilic attack a t a @-carbonatom. We are currently exploring the wider implications of these fascinating reactions.
Acknowledgment. We are grateful to the National Research Council of Canada for financial support of this work. Supplementary Material Available: A listing of structure factors for 1 -phenyl-2,3-bis(diphenylphosphino)naphthalene ((PhzP)zC16H10) and P ~ C ~ ~ ( P ~ ~ P C E C P ~ ) ~ .(29 ~ Cpages). H , C NOrdering inforrnation is given on any current masthead page.
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in cases of ~ e n t r a l and ~ . ~ a ~ i a l chirality ~.~ a t nickel, -23 kcal/mol(453 K) of free activation enthalpy2 being required for transformation of 1 (R'= C6H5) into its enantiomer 2. The C2 symmetry of these compounds may be easily recognized from projection formulae such as la and 2a, viewing 1 as the R configuration and 2 as the S in idealized tetrahedral coordination from the right-hand side. As previously e ~ p l a i n e d , ~ the more heavily substituted flanks of the chelate moieties were denoted as squares and the hydrogen sites as circles. Since very little is known about isomerization and substitution mechanisms of open-shell, tetrahedral transition-metal complexes, a determination of the barrier against racemization (1 2) would be of obvious i n t e r e ~ t Rather .~ than trying to
References and Notes (1) For example c-bis(phenylethyny1)benzene exhibits intramolecular interaction between the triple bonds in reactions with electrophiles or nucleophiles: H. W. Whitiock and P. E. Sandvick. J. Am. Chem. SOC.,88, 4525
(1 966). (2) Photochemical dimerizationpf ebis(phenylethyny1)benzeneyields verden, an azulene derivative, via intermolecular coupling: E. Muller, M. Sauerbier, D. Streichfuss, R. Thomas, W. Winter, G. Zountsas, and J. Heiss, Justus Liebigs Ann. Chem., 750 63 (1971). (3) D. K. Johnson and A. J. Carty, J. Chem. Soc., Chem. Commun., 903 (1977); A. J. Carty, S. E. Jacobson, R. T. Simpson, and N. J. Taylor, J. Am. Chem. SOC.,97, 7254 (1975). (4) Although chelating ligands with R2P or RzAs substituents on an aromatic backbone were extensively exploited in the renaissance period of inorganic chemistry, particularly by the groups of Chatt5 and Nyholm,6 their general utility has been severely hampered by the difficulty of their synthesis. Thus o-phenylenebis(diphenyiph0sphine) can be prepared in 1% overall yield from c-bromochiorobenzene in a tedious four-stage synthesis. See F. A. 3324 (1960). Hart, J. Chem. SOC., (5) J. Chatt, F. A. Hart, and H. R. Watson, J. Chem. SOC.,2537 (1962), and references therein. For a general review of bidentate phosphorus complexes, see W. Levason and C. A. McAuliffe, Adv. Inorg. Chem. Radiochem., 14, 173 (1972). (6) R. S.Nyhoim and G. J. Sutton, J. Chem. Soc., 560 (1958),and references therein. (7) Complexes 2 have characteristic v(M-CI) bands for cis stereochemistry (e.g.. 2 (M = Pt; R = R' = Ph), 314 (s), 294 (s); 2 (M = Pd; R = R' = Ph), v(Pd-CI) 321 (s), 300 (s) cm-') and have been analyzed satisfactorily. (8) This is a very simple and effective route to Pd(li) halo complexes which avoids the isolation of intermediates, e.g., PdCIz(PhCN)2, (C0D)PdCIz. (9) intensity data for 2 (M = pt; R = R' = Ph; X = Ci) were collected on a Sbntex P2' diffractometer using Mo Kcu radiation with a variable scan rate. A total of 5382 independent reflections were measured. For 3 a GE-XRD-6 Datex automated instrument operating in a 8/28 scan mode with Cu Kac radiation was used and 2433 independent measurements were collected. In both cases the structure was solved via Patterson and Fourier methods. Refinement was by full-matrix least-squares techniques. All nonhydrogen atoms have been refined using anisotropic thermal parameters.The residual R is defined as R = (ZI F, - F c l / Z I F,/). (10) Complex 4 (M = Pt; R = R' = Ph; X = CI)(O.5 g) and KCN (5 g) were refluxed overnight in benzonitriie(70 mL) under N2. Following evaporationof solvent, extraction with ether, dry chromatography on Fiorisil, and recrystallization from CH2C12-EtOH (1 : l ) , a mixture of needles (2,4,6-triphenyl-s-triazine from trimerization of PhCN, identified by mass spectrometry microanalysis, and comparison with an identical sample) and prisms 4 (-30% from 2) were obtained. ( 1 1) A. J. Carty, T. W. Ng. and G. J. Paienik, unpublished results.
resolve a racemic mixture, we chose to tackle this problem by the technique of chiral, nonracemizable anchor groups in 3. The required ligands were prepared from optically active (R)-camphor6 in the usual way.'-* In the isomer drawn as 3, the configuration a t nickel9 was denoted by the central R in R RR. Inversion a t the central metal will produce 4 with R S R configuration. Both of these diastereomers are chiral and of
L 1
3a
25 4,R1= CgH5 6 , R1-CH3
Arthur J. Carty,* Nicholas J. Taylor David K. Johnson Guelph- Waterloo Centre for Graduate Work in Chemistry Waterloo Campus, University of Waterloo Waterloo, Ontario, Canada N 2 L 3GI Received April 20, I979
exchange I
7
Sir:
Paramagnetic (dx), pseudotetrahedral' nickel N4 chelates of type 1 are configurationally stable on the N M R time scale.2 This surprising observation has been exploited for conformational analysis3 and for recognition of substituent rotations4 0002-7863/79/I501 -5424$01 .OO/O
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symmetry, as is easily seen from projections 3a and 4a where the wedges indicate orientations of the isopropylidene bridges. The overall structure is fully (pseud0)tetrahedral between -80 and +I37 "C in C12CD-CDCIl or tetralin as shown by the paramagnetism l oand temperature-independent reduced shifts.' I The interconversion of 3 and 4 was measured by integration of the two N M R signals for the two pairs of symmetry-related p-hydrogen atoms1*of 3 (phenyl groups a in 3a), as well as by C2
Barriers against Configurational Isomerization at Tetrahedral Nickel(I1)
4 a
0 I979 American Chemical Society
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Communications to the Editor
the observation of two new, corresponding signalsI2 of 4 ( b in Time-Resolved CIDEP and ESR Studies 4a). On the other hand, 3 and 4 behave like enantiomers in of Heavy Metal-Organic Radical Complexes. their chiroptical properties with respect to nickel, i.e. [0]630 ca. The Uranyl-Phenanthroquinone Radical Ions - 1 190 and 1260. Therefore, the diastereomerization 3 Sir: 4 can also be monitored by time-dependent circular dichroism (mutarotation). The interconversion rates are cleanly first The rather turbulent debate as to the origins of the CIDEP order for 3 in tetralin solution; both methods define a common (chemically induced dynamic electron polarization) observed Eyring plot with AH* = 22.5 (40.8) kcal/mol and AS* = in the ESR spectra of photogenerated radicals has abated, and, - 1 5 (f3)cal K-' mol-'. The AG* values of 27.0 (at room as it is now generally accepted that there are two distinctly temperature) or 29.3 kcal/mol (at 453 K for comparison with different mechanisms,' interest has turned to chemical ap1) thus confirm the lower limit2 of 321.8 kcal/mol for 1 ( R ' plications. While almost all of the CIDEP initial polarization = C6H5, R2 = CH3, R3 = C2H5). systems reported to date involve the photochemical triplet of Does the bulky bornane skeleton perturb such barriers? W e organic carbonyl compounds,l our recent efforts in CIDEP measured the inversion 5 + 6 for comparison with the known2 applications have been mainly directed towards metal-quinone activation enthalpy 17.3 kcal/mol (at 358 K in cyclohexane) complexes, particularly the o-phenanthroquinone (PQ).2 We of 1 (R' = R2 = CH3, R 3 = C2H5). Both processes are fast on report here our first successful application of time-resolved the N M R time scale and can be followed by coalescence CI DEP to heavy metal-organic radical complexes: the urastudies on several pairs of diastereotopic protons. Although 5-6 nyl-phenanthroquinone radical ions. Historically the photois too sensitive to be isolated in pure form, its very large and chemistry of uranyl ion has played an important role3 in the characteristic chemical ' H N M R shifts permit the easy evaldevelopment of modern photochemistry. The CIDEP results uation of AG* = 17 ( f l ) kcal/mol (at 345 K in benzenewill shed some light on the primary photochemical processes tetralin). Thus a t least for R' = CH3 the comparison of 5-6 of uranyl ions and the ESR characterization of the uranylwith the above 1 shows no distinct perturbation by the bornane quinone complex ions should be of wider interest to chemistry moiety. in general. Barriers of such heights are very unusual'3 for open-shell The laser flash photolysis (Molectron 1-MW N2 pulsed tetrahedra. When diastereomerization of 3 was performed in laser) and the time-resolved dc detection CIDEP observation the presence of racemic ligand, the third possible, completely system were assembled similarly to those reported by Kim and asymmetric (C, in 7a) diastereomer 7 was also o b ~ e r v e d ' ~ ~ W ' ~e i ~ s r n a n .The ~ total spectrometer d c response time was as R R S (and/or its antipode2 R S S ) . Both 4 and 7 N M R measured and found to be 0.2 ws. A detailed examination of signals appear with comparable rates but now somewhat faster the system performance and the analysis of the relaxation than in the absence of free ligand. Since 7 can be formed from measurements will be described elsewhere. 3 only by ligand exchange, it is clear that substitution can be When a degassed T H F containing M each of PQ and faster than configurational inversion. Some mechanistic feaUOr(N03)2-6H2O was exposed briefly to light in the ESR tures of these competing pathways will be published shortly. cavity with 100-kHz modulation a t -60 O C , a well-resolved spectrum was observed (Figure 1A). Prolonged UV irradiation led to the disappearance of the spectrum and in its place a new Acknowledgment. We are grateful to Professor E. Wunsch, Dr. E. Jaeger, and Dr. S . Knof (Abteilung Peptidchemie spectrum (Figure I B) was developed when irradiation was terminated. Both spectra are characterized by a distinctly low des Max-Planck-Instituts fur Biochemie, Martinsried bei g factor and their ESR parameters are given in Table I . The Munchen) for access to and intense technical assistance with analyses of the hyperfine structures are consistent with the the dichrographe. This research is supported by the Stiftung assignments of [UO2PQ]+. and [U02HPQl2+., respectively, Volkswagenwerk and the Deutsche Forschungsgemeinin which the unpaired spin is associated mainly with the pheschaft. nanthroquinone moiety. The low g factor can be explained by References and Notes the coordination to the uranium nucleus having a large spinorbit coupling. Hyperfine interaction due to 235U ( I = 7/2, Sheldrick. W. S.;Knorr. R.; Polzer, H. Acta CfystaIIogr., Sect. B 1979, 35,
+
+
739-741. Knorr, R.; Weiss, A.; Polzer, H.; Rapple, E. J. Am. Chem. SOC.1977, 99, 650-65 1. Knorr, R.; Weiss. A. J. Chem. Soc., Chem. Commun. 1977, 173-174, 392. Knorr, R.; Weiss. A.; Polzer, H. Tefrahedron Left. 1977, 459-462. Whitesides. T. H. J. Am. Chem. SOC. 1969, 91, 2395-2396. Elian, M.; Hoffmann, R. Inorg. Chem. 1975, 74, 1058-1076. Lohr. L. L. J. Am. Chem. SOC. 1978, 100, 1093-1099. R is used as a simplified notation for the 1R4R configuration of &(+)camphor. McGeachin, S.G. Can. J. Chem. 1968,46, 1903-1912. Structural assignments of these compounds were based on full spectroscopic characterization and correct elemental analyses. Configurational assignments at nickel are arbitrary but in no way relevant for the conclusions drawn here. Optically active 3 crystallized in pure form from ethyl acetate-ethanol: mp 187-189 OC;molecular mass (benzene)calcd 718, found 690; magnetic dipole moment, found 3.14 in (CIpCD)*at 26 O C . Knorr. R.; Polzer, H.;Bischler. E. J. Am. Chem. SOC. 1975, 97, 643644. The reduced NMR shifts" of these 4-H's are iY +25.7 and +29.3 ppm for 3, 4-27.0 and +28.3 ppm for 4, +26.4 and +30.5 ppm for 7. See ref 2 and 5 for some relevant references. The resultant mixture showed the same NMR shifts as a solution of the nickel complex8 (mp 237.5-239 "C)prepared from the racemic ligand.
Rudolf Knorr,* Friedrich Ruf Institute of Organic Chemistry. University of Munich Karlstr. 23, 8000 Munich 2, West Germany Received April 9, I979 0002-78631791 I501-5425$01 .OO/O
Figure 1. ESR spectra of ( A ) [UOzPQ]+ in T H F at -60 OC, (B) IUO?HPQ]?+i n T t I F a t -60 OC.
0 I979 American Chemical Society
